In the manufacture of tissue paper, often a through-air dryer is used to non-compressively dry the tissue paper to provide for a softer tissue paper. Since the tissue web is often molded by the through-air dryer fabric during the drying process, the design of the through-air dryer fabric has a large impact on the physical properties of the tissue paper. Often it is necessary to change the through-air dryer fabric to a different fabric when changing the grade of tissue paper being produced. Alternatively, the through-air dryer fabric needs to be changed when it becomes worn out or damaged.
In order to change the through-air dryer fabric, the stock feeding the paper machine must be diverted and the paper machine brought to a slower idling speed. The machine is then run at this slower speed until the through-air dryer and through-air dryer fabric cool sufficiently such that the paper machine can be brought to a stop without burning the fabric portion remaining within the through-air dryer. Once the through-air dryer is cool enough to work on, the fabric can be changed and the cycle reversed to bring the paper machine back on-line.
Because of the thermal mass involved, it takes a fairly long time to warm up or cool down the through-air dryer. Also, because of the design and materials used to make the through-air dryer, different parts of the dryer can take different lengths of time to warm up or cool down. The temperature differential and differences in the coefficient of thermal expansion between the different materials used in the construction of the through-air dryer can cause thermal stresses in the through-air dryer during warm up or cool down. To limit the induced thermal stresses, the warm up or cool down duration is often increased beyond the minimum time period it would take to warm up or cool down the through-air dryer if done as fast as possible. Increasing the warm up or cool down period represents a significant loss in productivity every time the through-air dryer fabric needs to be changed. Therefore, what is needed is a faster way to warm up and cool down a through-air dryer in order to speed up a fabric change while limiting the maximum thermal stresses to current or lower levels.